The invention relates to fluid sensors and particularly to microsensors. More particularly it pertains to housing for such sensors. Microsensors have one vexing packaging problem. Their space-saving and cost-saving smallness, in surface area and membrane thickness, make them vulnerable to occasional impacts with particles. The solid particles may destroy the gas-sensitive membrane of the sensor or change its heat transfer features with just a thin coat of sticky particles. The liquid ones may have the same effect as the small solids, if a residue stays after re-evaporation.
These problems continue to be of concern in relation to the development of microsensors of fluid vapor as needed for control or recovery operations of such vapors. The cause of the problem is the need to satisfy two competing goals which are to achieve a short response time (e.g., one to three seconds or less) and a service life of about ten years. Resolving the fundamental approach to sensing fluid properties is an important and essential step. But at least of equal importance is the design of a sensor housing or package, which will enable the sensor to perform its function rapidly, sensitively and reliably, even in harsh environments. The problem is that the filters and baffles one would provide to insure protection for long and reliable sensor service are the same that would increase response time to unacceptable levels. The present invention provides a solution and tradeoff between speed of response and sensor protection.
The present microsensor housing both protects small, one micrometer-thick sensing structures of thermal microsensors, and facilitates rapid and reliable operation in spite of exposure to forced convection, flow turbulence, dust, droplets and/or condensation.
In order to sense fluid (i.e., gas or liquid) thermophysical properties such as thermal conductivity, specific heat, or its derivatives of oxygen demand, heating value, compressibility factor or octane number, the sensor needs to be in contact with the fluid and be able to reliably sense small changes in the above properties. The sensitivity is provided by the design of the sensor as a chip, featuring low mass, large surface-to-volume ratio heating and sensing elements. Long and reliable service requires that the sensor be protected from interference due to settling dust or droplets, as well as from flow (laminar or turbulent). Protection against condensation means that the sensor is designed to recover its sensing performance within a specified short time after coming in contact with liquid condensates. Rapid response means that the sensor chip itself needs to respond quickly to changes in the fluid properties, as well as that the sensor housing needs to allow quick transport and replacement of xe2x80x9coldxe2x80x9d with xe2x80x9cnewxe2x80x9d fluid sample elements, without noticeable thermal disturbance due to forced convection or turbulence.
For a fluid property sensor to meet a specified microsensor performance in terms of response time, insensitivity to flow, and service life, aspects of four parameter groups, which a designer can adjust to meet the desired sensor performance, include sensor chip design and performance, geometry of the convective transport section of the sensor housing, geometry of the convective barrier, and geometry of the diffusion transport section.
For the parameters of this tool kit, there are generic as well as quantitative guidelines for the design of microenvironmental protection of (thermophysical property) microsensors, to meet conflicting performance demands for xe2x80x9cfast responsexe2x80x9d, xe2x80x9coperability in high flowsxe2x80x9d and xe2x80x9clong, reliable servicexe2x80x9d in harsh field environments. These were characterized by their average dust loads, occasional condensation, maximum flow velocities and flow turbulence, which had resulted in slow response time problems before, due to excessive protection. As a result of this invention, the specifying of performance (response time and service life), and the characterizing of environmental conditions, a microsensor housing for both property sensors and flow sensors has been developed which enables the packaged sensor to meet the desired performance and lifetime specifications.
The proposed approach is shown in FIGS. 4c and 4e. The single-stage baffle is shaped to facilitate liquid runoff via the sides, if liquid should get near the sensor chip. It is machined with a set of concentric holes projecting an area around the chip and inhibiting direct splashes from the direction of the fitting to hit the chip. It provides chip protection while allowing diffusional access of fluid to the chip from all sides.
In summary, the disclosed housing for microsensors features a new environmental protection design based on a single-stage, concentric baffle with openings arranged around the protected sensor. It minimizes remaining dead spaces around the sensor (to reduce response time) by filling-in those spaces that are non-essential for fluid diffusion.
There are advantages of the invention relative to prior art screens and non-concentric baffles. It can be machined in one piece. Its baffle does not need assembly after machining. The concentric baffle holes are large enough to make the probability of clogging negligible. The response time is five to nine times smaller than that measured with a previous 2-stage, non-concentric baffle (having two offset and opposed D-shaped louvers of FIG. 1b). The housing orientation relative to external flow direction does not affect the baffle""s effectiveness. It is easy to machine, requires no assembly, and barely increases microsensor response time relative to not having a baffle at all.